The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Traditionally, there have been two primary forms of reciprocating piston or rotary internal combustion engines: diesel and spark ignition engines. While these engine types have similar architecture and mechanical workings, each has distinct operating properties. For example, to initiate combustion, spark ignition engines supply an air/fuel mixture to the engine cylinder while controlling spark timing. In contrast, diesel engines compress air in the cylinder while controlling fuel injection timing to initiate the start of combustion.
One of the major advantages that the diesel engine has over the pre-mixed charge spark-ignited engine is higher thermal efficiency. This is generally due to the higher compression ratio and leaner combustion operation provided by the diesel engine. One trade-off to the higher thermal efficiency of the diesel engine is that it is more difficult or expensive to achieve the same tailpipe NOx emission levels as does the spark-ignited engines. This is due to the lean air/fuel control nature of the diesel engine.
Premixed Compression Ignition (PCI) is an advanced diesel combustion technique that has great potential for reducing diesel engine emissions. With PCI, fuel is injected into the combustion chamber of the cylinder much earlier in the combustion stroke than would be done for diesel combustion. The desired fuel amount is supplied significantly before the piston reaches the compression top dead center (TDC). The early injected fuel is mixed sufficiently with the air before the piston reaches the compression TDC. Thus, the technique provides a lean and well mixed state of the air/fuel mixture before ignition.
However, PCI combustion is limited to low-load operating conditions. Therefore, during other operating conditions diesel combustion is required. Because PCI combustion and diesel combustion have different requirements for the exhaust gas recirculation (EGR) percentage, the air/fuel ratio, and the fuel injection timing, the problem of how to switch smoothly between these two combustion modes becomes a concern. Excessive smoke, NOx, and combustion noise will result from lack of effective combustion mode switching control.